1. Field of the Invention
The present invention relates to an error correctable data transmission system, and in particular is directed to a system for transmitting digital information signals through a transmission medium such as magnetic tape or an optical digital audio disc, where the transmitted signal is subjected to disturbances causing both burst errors and random errors, and for enabling correction, with high error-correcting ability, of the errors which occur in the received digital signal.
2. Description of the Prior Art
There has been previously proposed, for example, in copending application Ser. No. 218,256, filed Dec. 19, 1980 and having a common assignee herewith, a data transmission system effective for correcting burst errors using a so-called cross-interleave technique. In such cross-interleave technique, words in a PCM (pulse code modulated) data signal are provided in plural sequences on plural respective channels arranged in a first arrangement state, and are furnished to a first error-correcting coder to generate therefrom a first check word series. This first check word series and the PCM data signal series in the plural channels are converted to a second arrangement state. Then, one word in the second arrangement state for each of the PCM data signal sequences in the plural channels is furnished to a second error correcting coder to generate therefrom a second check word series, so that a double interleave (i.e., double re-arrangement) is carried out for each word. The purpose of the double interleave is to reduce the number of erroneous words in any group of words contained in a single error-correcting block when the check word contained in such error-correcting block and the PCM data associated therewith are transmitted. Any such erroneous words are dispersed among several blocks, and are returned to the original arrangement thereof at the receiving side. In other words, when a burst error develops during transmission, the burst error can be dispersed. If the above interleave is performed twice, the first and second check words each are used to correct errors in distinct error correcting blocks. Thus, even if an error cannot be corrected by one of the first and second check words, the error can be corrected by the other check word. Therefore, this technique provides a significant advance in error correcting ability for burst errors.
However, when even one bit in one word is discovered to be in error, the entire word is considered erroneous. Therefore, when a received data signal has a relatively large number of random errors, the above-described double interleave technique is not always sufficiently powerful for correcting these random errors.